1. Field of the Invention
The present invention relates generally to a composite magnetic body, further to a magnetic element such as an inductor, a choke coil, a transformer, or the like. Particularly, the present invention relates to a miniature magnetic element used under a large current and a method of manufacturing the same.
2. Related Background Art
With the reduction in size of electronic equipment, the reduction in size and thickness of components and devices used therein also has been demanded strongly. On the other hand, LSIs such as a CPU are used at higher speed and have higher integration density, and a current of several amperes to several tens of amperes may be supplied to a power circuit provided in the LSIs. Hence, similarly in an inductor, size reduction has been required, and in addition, it has been required to suppress heat generation caused by lowering the resistance of a coil conductor, although that is contrary to the size reduction, and to prevent the inductance from decreasing with DC bias. The operation frequency has come to be higher and it therefore has been required that the loss in a high frequency area be low. Furthermore, in order to reduce the manufacturing cost, it also has been requested that component elements with simple shapes can be assembled in easy processes. In other words, there has been demand for a miniaturized thinner inductor that can be used under a large current and at a high frequency and can be provided at low cost.
With respect to a magnetic body used for such an inductor, DC bias characteristics are improved with the increase in saturation magnetic flux density. Higher magnetic permeability allows a higher inductance value to be obtained but tends to cause magnetic saturation and thus, the DC bias characteristics are deteriorated. Hence, a desirable range of the magnetic permeability is selected depending on the intended use. In addition, it is desirable that the magnetic body have higher electrical resistivity and lower magnetic loss.
Magnetic materials that have been used practically are divided broadly into two types of ferrite (oxide) materials and metallic magnetic materials. The ferrite materials themselves have high magnetic permeability, low saturation magnetic flux density, high electrical resistance, and low magnetic loss. The metallic magnetic materials themselves have high magnetic permeability, high saturation magnetic flux density, low electrical resistance, and high magnetic loss.
An inductor that has been used most commonly is an element including an EE- or EI-type ferrite core and a coil. In this element, a ferrite material has high magnetic permeability and low saturation magnetic flux density. When the ferrite material is used without being modified, the inductance is decreased considerably due to the magnetic saturation, resulting in poor DC bias characteristics. Therefore, in order to improve the DC bias characteristics, usually such a ferrite core and a coil have been used with a gap provided in a magnetic path of the core to decrease the apparent magnetic permeability. However, when such a gap is provided, the core vibrates in the gap portion when being driven under an alternating current and thereby noise is generated. In addition, even when the magnetic permeability is decreased, the saturation magnetic flux density remains low. Consequently, the DC bias characteristics are not better than those obtained using metallic magnetic powder.
For example, a Fe—Si—Al based alloy or a Fe—Ni based alloy having higher saturation magnetic flux density than that of ferrite may be used as the core material. However, because such a metallic material has low electrical resistance, the increase in high operation frequency to several hundreds of kHz to MHz as in the recent situation results in the increase in eddy current loss and thus the inductor cannot be used without being modified. Accordingly, a composite magnetic body with magnetic powder dispersed in resin has been developed. The composite magnetic body can contain a coil. Hence, a larger cross sectional area of magnetic path can be obtained when using such a composite magnetic body.
In the composite magnetic body, an oxide magnetic body (ferrite) with high electrical resistivity may be used as a magnetic body. In this case, because the ferrite itself has high electrical resistivity, no problem is caused when a coil is contained in the composite magnetic body. However, when using the oxide magnetic body that cannot be deformed plastically, it is difficult to increase its packing ratio (filling rate). In addition, the oxide magnetic body inherently has a low saturation magnetic flux density. Thus, sufficiently good characteristics cannot be obtained even when the coil is embedded. On the other hand, when using metallic magnetic powder that can be deformed plastically and has high magnetic saturation flux density, the electrical resistivity of the metallic magnetic powder itself is low, and therefore the electrical resistivity of the whole magnetic body decreases due to contacts between powder particles with the increase in packing ratio. As described above, there has been a problem that the conventional composite magnetic body cannot have sufficiently good characteristics while maintaining high electrical resistivity.